Frequency quality criteria for inter-frequency handover in a TD-CDMA communication system

ABSTRACT

A method for determining whether to perform IFHO (inter-frequency handover) of a UF apparatus from one cell to another and from one carrier frequency to another in a TD-CDMA telecommunications network (including e.g. a TD-SCDMA network)—based separately on coverage and capacity, using one decision metric for coverage (e.g. based on received signal code power for each candidate cell and each candidate carrier frequency) and a different decision metric for capacity (e.g. based on a quantity proportional to the received signal code power and inversely proportional to the difference between a received signal strength indicator and the received signal code power). Corresponding equipment and a computer program product is also provided. In deciding whether IFHO is needed based on capacity, a threshold for received signal code power may be used in addition to the metric for capacity.

TECHNICAL FIELD

The present invention pertains to the field of wireless communication. More particularly, the present invention pertains to criteria used in inter-frequency handover in a cellular communication system.

BACKGROUND ART

TD-SCDMA (Time Division-Synchronized Code Division Multiple Access), also known as UTRA (Universal mobile telecommunications system (UMTS) Terrestrial Radio Access) TDD (time division duplex) 1.28 Mcps option, is a so-called 3G (third generation) wireless communication system that is supposed to be used for example in China, possibly along with two other 3G technologies—UTRA FDD version of WCDMA (Wideband CDMA) and CDMA2000.

TD-SCDMA uses both TDMA (Time Division Multiple Access) and CDMA as multiple access methods, which means there are several possible slots for each transmitting direction on a single carrier, and in each slot one or more users, separated by orthogonal codes, can transmit or receive data simultaneously, one user can even occupy more than one timeslot during multi-slot mode.

Since TD-SCDMA uses a 1.6 MHz bandwidth, it has a low single-carrier capacity. Thus, a TD-SCDMA is usually implemented (by a network operator) using more than one carrier. Even so, to prevent a communication link from deteriorating because of over-capacity, inter-frequency handover (IFHO) must be performed relatively frequently. IFHO is also performed because of coverage, i.e. because of a user equipment (UE) (a cellular communication device, e.g. a cellular telephone) moving beyond the coverage of a network service access point (SAP), e.g. a so-called Node B.

TD-SCDMA uses hard handover (from one SAP to another, without a time interval during which the UE is in communication with both) instead of soft handover (includes a time interval during which the UE is in communication with two or more SAPs) to make the UE always connect to a single best cell. The quantity used to define how good a cell is, per the prior art, P-CCPCH RSCP (Primary-Common Control Physical CHannel Received Signal Code Power). After passing the cell-planning phase, the coverage area is decided by the antenna down tilt, gain map and Tx (transmit) power of P-CCPCH. The Tx power of P-CCPCH changes only rarely. Hence the coverage area of a cell also rarely changes. The cell-planning phase ensures that the best cell's P-CCPCH RSCP in the whole service area is above an acceptable threshold.

Based on where the UE is located in a service area, the P-CCPCH RSCP of all detectable cells would reflect the admission cost (power increase of the whole network for both directions) of the UE in that location. Estimation cost in slot 1 to slot 6 from measurement of slot 0, which is the slot used by P-CCPCH, would not be very accurate, but it would be sufficient on an average basis without information about other slots in the location. The potential downlink power increase after admitting the UE in the specific location is decided by both the path loss from the serving cell and the path loss from any interfering cells. The serving cell's P-CCPCH RSCP would only directly reflect the path loss from the serving cell, but it would also indirectly reflect the path loss from interfering cells. In other words, the serving cell's P-CCPCH RSCP is affected by and so indicates interference from interfering cells. So selecting a cell having the best P-CCPCH RSCP in one carrier as the serving cell is a UE's best choice for intra-frequency handover.

However, for inter-frequency handover cell quality criteria, selecting a cell having the best P-CCPCH RSCP is not always the best choice since a good P-CCPCH RSCP does not necessarily indicate low interference at slot 0 compared with other cells.

Thus, what is needed is a new decision algorithm by which to select a best cell during inter-frequency handover based on minimizing admission cost.

DISCLOSURE OF THE INVENTION

Accordingly, in a first aspect of the invention, a method (for use by e.g. a network element of a wireless communication system) is provided comprising: a step of selecting either a decision metric related to coverage or a decision metric related to capacity as a basis for deciding whether to perform an inter-frequency handoff of a user equipment apparatus; and a step of determining whether to perform the interfrequency handover using the selected decision metric.

In accord with the invention, the decision metric related to capacity may vary in inverse proportion to the difference between a received signal strength indicator for a cell and a carrier frequency and the received signal code power for the cell and the carrier frequency, or in inverse proportion to quantities algebraically related so as to behave in a similar way.

Also in accord with the invention, the decision metric related to capacity may be the received signal code power for a cell and a carrier frequency divided by the difference between a received signal strength indicator for a cell and a carrier frequency and the received signal code power for the cell and the carrier frequency, or may be a quantity algebraically related so as to behave in a similar way.

In accord with the first aspect of the invention, the step of determining whether to perform an interfrequency handover may be based not only on the decision metric related to capacity, but also on a threshold for received signal code power.

In a second aspect of the invention, a computer program product is provided comprising a computer readable storage structure embodying computer program code thereon for execution by a computer processor, where the computer program code comprises instructions for performing a method according to a first aspect of the invention.

In a third aspect of the invention, a network element is provided, comprising: means for selecting either a decision metric related to coverage or a decision metric related to capacity as a basis for deciding whether to perform an inter-frequency handoff of a user equipment apparatus; and means for determining whether to perform the interfrequency handover using the selected decision metric.

In a fourth aspect of the invention, a method (for use e.g. by a user equipment apparatus) is provided, comprising: a step in which a user equipment apparatus receives an indication that an inter-frequency measurement is to be performed and that an indicated decision metric is to be used in ranking cells and frequencies; and a step in which the user equipment apparatus performs an interfrequency measurement and ranks the results for different cells and frequencies according to the decision metric; wherein the decision metric can be indicated to be either a decision metric related to capacity or a decision metric related to coverage.

In a fifth aspect of the invention, a computer program product is provided, comprising a computer readable storage structure embodying computer program code thereon for execution by a computer processor, where the computer program code comprises instructions for performing a method according to the fourth aspect of the invention.

In a sixth aspect of the invention, a user equipment apparatus is provided, comprising: means by which the user equipment apparatus receives an indication that an inter-frequency measurement is to be performed and that an indicated decision metric is to be used in ranking cells and frequencies; and means by which the user equipment apparatus performs an interfrequency measurement and ranks the results for different cells and frequencies according to the decision metric; wherein the decision metric can be either a decision metric related to capacity or a decision metric related to coverage.

In a seventh aspect of the invention, a system is provided, comprising: a network element according to the third aspect of the invention, and a user equipment apparatus coupled to the network element via a radio link. The user equipment apparatus may be according to the sixth aspect of the invention. The radio link may be e.g. any kind of time division code division multiple access radio link including a time division synchronized code division multiple access radio link, or a time division duplex radio link.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the invention will become apparent from a consideration of the subsequent detailed description presented in connection with accompanying drawings, in which:

FIG. 1 is a block diagram showing a system of a type for which the invention can be used, and showing a network entity—and in particular a radio network controller—including means for performing a method according to the invention.

FIG. 2 is a flow chart of a method according to the invention (for execution by a network entity, such as a radio network controller).

FIG. 3 is a flow chart showing operation of a UE according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the IFHO algorithm for TD-SCDMA is modified so as to include a frequency criteria or metric for two different causes of IFHO: one for coverage-caused link deterioration (i.e. for the case that a UE moves outside of the coverage area of a SAP) and the other for over-capacity. For coverage-caused IFHO, the IFHO metric can be the same as in the prior art, namely: M ^((IFHO-coverage)) =P _(RSC),   (1) where P_(RSC) is P-CCPCH RSCP (for a particular cell i, not made express in the above, and for a particular frequency j, also not made express), i.e. the received signal code power for P-CCPCH (for a cell i). For over-capacity, the IFHO metric can be: M ^((IFHO-capacity)) =P _(RSC)/(I _(RSS) −P _(RSC)),   (2) (again for a particular cell i, not made express, and for a particular frequency j, also not made express) where I_(RSS)=UTRA Carrier RSSI (Received Signal Strength Indicator), which is used here to indicate the wideband power of slot 0. Other metrics for the IFHO capacity metric are possible according to the invention, but advantageously, any IFHO capacity metric exhibits the same general behavior as M^((IFHO-capacity)) given by eq. (2) by virtue of being algebraically related. Thus, for example, the invention includes as possible formulae for M^((IFHO-capacity)) any of the following: [P_(RSC)/(I_(RSS)−P_(RSC))]², P_(RSC)/(I_(RSS)−P_(RSC)) P_(RSC)/(I_(RSS)−P_(RSC))², P_(RSC)/(I_(RSS)−P_(RSC) ²), log {P_(RSC)/(I_(RSS)−P_(RSC))}, or P_(RSC)/(I_(RSS)−P_(RSC+)δ), where δ is a small constant for regulating the behavior of M^((IFHO-capacity)) when I_(RSS)−P_(RSC) is small. Other formulae are of course also possible. According to the invention, any formula for M^((IFHO-capacity)) takes into account both PRSC as well as how close P_(RSC) is to I_(RSS).

Thus, according to the invention, the decision algorithm used to decide whether to perform IFHO tests for both over-capacity and coverage based link deterioration, and performs IFHO in case either test indicates that IFHO should be performed. For the capacity test, the best cell i (and for a particular frequency j)—and so the cell to which handover would be performed—can be the cell having the highest M^((IFHO-capacity)) and also having an acceptable P-CCPCH RSCP, i.e.: $\begin{matrix} {{\underset{i,j}{Max}\left\{ M_{i,j}^{({{IFHO} - {capacity}})} \right\}},} & (3) \end{matrix}$ where i indicates a neighboring cell and j the frequency, and P_(RSC) ^(i,j)>P_(RSC) ^(threshold)   (4) where P_(RSC) ^(threshold) is a P-CCPCH threshold designed to be several dB (deciBel) above a minimum required (e.g. by an accepted standard) P-CCPCH value. The threshold requirement given by eq. (4) is to avoid link deterioration because of coverage reason soon after a UE is handed over to a cell because of the limit on capacity in the cell from which the UE is handed over.

For an IFHO decision because of coverage, the best cell can be the cell having highest P-CCPCH RSCP, as in the prior art, or having the largest M^((IFHO-coverage)) value for any IFHO coverage metric (typically having the same general behavior as the quantity P-CCPCH RSCP).

A UE is advantageously (although not necessarily) adapted for use according to the invention by including in it functionality for ranking cells according to the both kinds of metrics described above. According to the prior art (3GPP TS 25.311, section 10.3.7.44), the measurement report sent from the UE to the RNC should rank cells from best to worst according to RSCP, as measured by the coverage metric, and so a UE according to the invention should include functionality for ranking cells in two ways: according to the coverage metric for coverage-caused IFHO and according to the capacity metric for capacity-caused IFHO.

The RNC advantageously then commands a UE to perform a frequency quality measurement and provide results either according to a capacity metric or a coverage metric, as described above. The downlink Ec/No according to the prior art (3GPP TS 25.311, section 14.2.0a) can be re-interpreted to indicate to the UE that it is to use a capacity metric as described above (e.g. P-CCPCH RSCP/UTRA Carrier RSSI (slot 0)) for the UTRA TDD 1.28 Mcps option. With such a change, when the measurement quantity (as indicated by FE-TDD where FE is the Frequency Quality Estimate Quantity) is set to primaryCCPCH-Ec-NO (as opposed to RSCP or path loss), the UE should use the capacity metric. When it is set to RSCP, the UE should use the coverage metric.

In other embodiments, the RNC receives from the UE the raw measurement results and itself performs the rankings according to either a coverage metric or a capacity metric. In such embodiments, the UE need not be at all changed to be used with the invention.

A decision algorithm according to the invention advantageously resides in the RRM/HC (Radio Resource Management/Handoff Control) of the RNC (Radio Network Controller). The quantities P-CCPCH RSCP and UTRA Carrier RSSI are already supported in the RRC (Radio Resource Control) measurement report message specified by 3GPP TS 25.331, v500. Page 841 of 3GPP TS 25.331 v500 gives the following as a frequency quality metric for TDD (Time Division Duplex) used in determining whether to perform IFHO: Q _(i,j)=10·Log(M _(i,j))+O _(i,j)   (5) where Q_(i,j) is the estimated quality of cell i on frequency j, M_(i,j) is the measurement result for Primary CCPCH RSCP of cell i on frequency j expressed in mW, and O_(i,j) is the cell individual offset (set by the information element (IE) called “cell individual offset”) of the currently evaluated cell i on frequency j. The invention can be implemented to use the frequency quality metric given by eq. (5), but using two different M_(i,j): M_(i,j) ^((IFHO-coverage)) for coverage, and M_(i,j) ^((IFHO-capacity)) for capacity, as explained above.

Thus, and now referring to FIG. 1, according to the invention, a UE 11 is physically located so as to be able to establish wireless communication with a radio network controller 14 a and on to a core network 16 (and ultimately another communication terminal or server, not shown) via a radio link to a SAP 12 a having a cell i and using a carrier frequency j from among the various carrier frequencies 1, . . . , j, . . . , N available for use with the SAP 12 a. An IFHO (hard handover, in which the carrier frequency changes) can be performed to another SAP 12 b-d as needed because of deterioration in the radio link due to either the UE having moved outside of the cell i (or having moved to a location where coverage from another of the SAPs 12 b-d is superior) or because of too many users in the cell i. In case of an IFHO, the UE can be handed over to another of the SAPs 12 b-d (each providing a different cell than cell i) and the carrier frequency is changed to another of the available carrier frequencies (depending on the SAP selected as the serving SAP), according to the invention as described above, i.e. using a metric for coverage and a metric for capacity.

As indicated above, a radio access network (the RNCs and SAPs) can have several different carriers to use, and can use some in one cell and some (the same or different) in others of the cells. Thus, the carrier frequencies for two cells may overlap partially, wholly, or not at all. Typically, though, at least one carrier frequency is used in all of the cells in order to provide for continuous coverage in the service area, and other carrier frequencies are used in addition only where demand is high. (Note also that the coverage areas of the different carrier frequencies for a cell need not be the same within the cell.)

Referring now also to FIG. 2, the process of deciding whether to perform IFHO—and if so to what cell and carrier frequency—is shown as including a first step 21 in which the RNC obtains from UE measurement results needed to decide whether to perform IFHO (for example, using messaging according to the prior art). In a next step 22, the RNC determines whether to perform IFHO based on coverage, and if so, to what cell and what frequency, using the measurement results provided by the UE. This determination can be made as in the prior art, e.g. as set out in 3GPP TS 25.331, v.500 (see page 841). If the outcome of the test indicates that IFHO should be performed, then in a next step 23, the IFHO is performed so as to possibly hand off the UE 11 to another of the SAPs 12 b-d, and so as to change the carrier frequency. If the measurements instead indicate that IFHO is not needed based on coverage, then in a next step 24, the RNC determines whether to perform IFHO based on capacity, and if so, to what cell and what frequency, using e.g. eqs. (3) and (4), but at any rate using a different metric than that used for determining whether IFHO is needed based on coverage. If the measurements indicate that IFHO is needed based on capacity, then the step 23 of performing IFHO is performed. Clearly, the determining of whether to perform IFHO based on coverage may be performed either before or after the determining of whether to perform IFHO based on capacity, and the order is advantageously tailored to each RNC 14 a-b.

As explained above, the invention provides a method and it also provides corresponding equipment consisting of various modules providing the functionality for performing the steps of the method. The modules may be implemented as hardware, or may be implemented as software or firmware for execution by a processor. In particular, in the case of firmware or software, the invention can be provided as a computer program product including a computer readable storage structure embodying computer program code—i.e. the software or firmware—thereon for execution by a computer processor.

Referring now to FIG. 3, in embodiments in which the UE itself performs rank orderings of cells and frequencies according to a decision metric, the invention provides a method according to which the UE is operative and including a first step 31 in which the UE receives from the RNC a command to perform an inter-frequency measurement and to then rank cells and frequencies based on an indicated decision metric that can be either coverage-related or capacity-related. In a next step 32, the UE performs the inter-frequency measurement, ranks the cells and frequencies based on the indicated decision metric, and reports the ranking to the RNC.

As indicated above, the invention is especially advantageous in case of TD-SCDMA. More generally, the invention is of use in any TD-CDMA (Time Division-Code Division Multiple Access) communication system, and so e.g. can also be used for the UTRA TDD (time Division Duplex) 3.84 Mcps option.

Advantages of the invention are that it takes into account the possible cite density differences of different frequencies, that the capacity caused IFHO tends to select a carrier in which cites are sparse, and that since MUD can cancel most of the intra-cell interference, the single cell capacity of sparse cites is much larger than that of a continuous coverage carrier. Further, the intra-frequency handover algorithm is unaffected by the invention, and also the measurement control and measurement report in inter-frequency measurement need not change.

It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the scope of the present invention, and the appended claims are intended to cover such modifications and arrangements. 

1. A method, comprising: a step of selecting either a decision metric related to coverage or a decision metric related to capacity as a basis for deciding whether to perform an inter-frequency handoff of a user equipment apparatus; and a step of determining whether to perform the interfrequency handover using the selected decision metric.
 2. A method as in claim 1, wherein the decision metric related to capacity varies in inverse proportion to the difference between a received signal strength indicator for a cell and a carrier frequency and the received signal code power for the cell and the carrier frequency, or in inverse proportion to quantities algebraically related so as to behave in a similar way.
 3. A method as in claim 1, wherein the decision metric related to capacity is the received signal code power for a cell and a carrier frequency divided by the difference between a received signal strength indicator for a cell and a carrier frequency and the received signal code power for the cell and the carrier frequency, or is a quantity algebraically related so as to behave in a similar way.
 4. A method as in claim 1, wherein the step of determining whether to perform an interfrequency handover based on capacity is based not only on the decision metric related to capacity, but also on a threshold for received signal code power.
 5. A computer program product comprising a computer readable storage structure embodying computer program code thereon for execution by a computer processor, wherein said computer program code comprises instructions for performing a method including: a step of selecting either a decision metric related to coverage or a decision metric related to capacity as a basis for deciding whether to perform an inter-frequency handoff of a user equipment apparatus; and a step of determining whether to perform the interfrequency handover using the selected decision metric; and wherein the decision metric related to capacity differs from the decision metric related to coverage.
 6. A network element, comprising: means for selecting either a decision metric related to coverage or a decision metric related to capacity as a basis for deciding whether to perform an inter-frequency handoff of a user equipment apparatus; and means for determining whether to perform the interfrequency handover using the selected decision metric.
 7. A method, comprising: a step in which a user equipment apparatus receives an indication that an inter-frequency measurement is to be performed and that an indicated decision metric is to be used in ranking cells and frequencies; and a step in which the user equipment apparatus performs an interfrequency measurement and ranks the results for different cells and frequencies according to the decision metric; wherein the decision metric can be indicated to be either a decision metric related to capacity or a decision metric related to coverage.
 8. A method as in claim 7, wherein the decision metric related to capacity varies in inverse proportion to the difference between a received signal strength indicator for a cell and a carrier frequency and the received signal code power for the cell and the carrier frequency, or in inverse proportion to quantities algebraically related so as to behave in a similar way.
 9. A computer program product comprising a computer readable storage structure embodying computer program code thereon for execution by a computer processor, wherein said computer program code comprises instructions for performing a method including: a step in which a user equipment apparatus receives an indication that an inter-frequency measurement is to be performed and that an indicated decision metric is to be used in ranking cells and frequencies; and a step in which the user equipment apparatus performs an interfrequency measurement and ranks the results for different cells and frequencies according to the decision metric; wherein the decision metric can be indicated to be either a decision metric related to capacity or a decision metric related to coverage.
 10. A user equipment apparatus, comprising: means by which the user equipment apparatus receives an indication that an inter-frequency measurement is to be performed and that an indicated decision metric is to be used in ranking cells and frequencies; and means by which the user equipment apparatus performs an interfrequency measurement and ranks the results for different cells and frequencies according to the decision metric; wherein the decision metric can be either a decision metric related to capacity or a decision metric related to coverage.
 11. A user equipment apparatus as in claim 10, wherein the decision metric related to capacity varies in inverse proportion to the difference between a received signal strength indicator for a cell and a carrier frequency and the received signal code power for the cell and the carrier frequency, or in inverse proportion to quantities algebraically related so as to behave in a similar way.
 12. A system, comprising: a network element as in claim 6, and a user equipment apparatus coupled to the network element via a radio link.
 13. A system as in claim 12, wherein the user equipment apparatus comprises: means by which the user equipment apparatus receives an indication that an inter-frequency measurement is to be performed and that an indicated decision metric is to be used in ranking cells and frequencies; and means by which the user equipment apparatus performs an interfrequency measurement and ranks the results for different cells and frequencies according to the decision metric; wherein the decision metric can be either the decision metric related to capacity or the decision metric related to coverage.
 14. A system as in claim 12, wherein the radio link is a time division code division multiple access radio link.
 15. A system as in claim 12, wherein the radio link is a time division synchronized code division multiple access radio link.
 16. A system as in claim 12, wherein the radio link is a time division duplex radio link. 